Method for producing adherent coatings on iron and steel parts



1965 H. GERISCHER ETAL 3,202,551

METHOD FOR PRODUCING ADHERENT COATINGS ON IRON AND STEEL PARTS Filed Sept. 7, 1962 SUQFACE CONMC T WITH OIL UTE HYDEOXY CAQBOXYZIC ACID 517125405 CONMCT WITH ANAQUEOLZS SOLUTION 0f CHIQOMC ACID SW62? EINSING WYTH WATER 01? W19? AQUEOUS 501 UTION Of 611290147 C ACID United States Patent M 3,2il51 IViETHGE) FER 653A!- ENGS 9N AND STEEL PART Heinz Gerischer, Stuttgart-Vaihingen, Witty iiierbst, Hofheirn, 'launns, Heinz Ludwig, ldraniti'urt am Main, Helmnt Schaeier, Hoiheim, Tanners, and Ernst Wagner, Frankfurt am Main, Germany, assignors to Farbwerlre Hoechst Alrtiengeselischatt vornials Rleistcr Lucius 8: Ernning, Frankfurt am Main, Germany, a corporation of Germany Fiied Sept. 7, llQZ, Ser. No. 222,195 Claims priority, application *Gerrnany, Sept. 13, 1961, F 34313 11 Claims. (Ci. l486.2)

The present invention relates to a method for producing adherent uniform coating on iron and steel parts, which serve in particular to prevent corrosion and to improve the adhesion of lacquers and varnishes. The invention is in particular concerned with the coating of iron and steel parts which is brouhgt about by contacting these parts in a first step with an aqueous solution containing at least one aliphatic hydroxycarboxylic acid as an essential component and in a second step with an aqueous solution containing chromic acid as an essential component, and by rinsing the metal parts thus treated in a subsequent step.

To protect iron and steel surfaces against corrosion it has already been proposed to treat them with solutions of acid compounds of hexavalent chromium without previous phosphatization.

According to one such proposal, the metals, freed from water deposits, are treated with a solution containing a chromate or chromic acid, an organic reducing agent for the chromate, phosphoric acid and a hydrogen phosphate of a bivalent metal ion. In the film of solution adhering to the surface the oxidizing and reducing substances are destroyed by heating, so that finally a coating is formed that is composed of approximately equal parts of chromium hydroxide and phosphate.

According to another proposal the metal parts are treated in a pickling bath comprising a solution, almost heated to boiling, of chromic acid or a salt thereof, to which is added a colloidal sulfur compound splitting oil sulfur and/ or an oxidizing agent. The addition of other reducing agents, such, for example, as sugar, to solutions of chromic acid of 0.5 to strength, has also already been proposed, the metal parts being previously treated with nitric acid of 1 to 20% strength.

Whereas, according to the aforementioned processes for treating iron and steel surfaces solutions are used that contain, besides compounds or" hexavaient chromi um, additions required for the formation of a coating, such, for example, as a phosphate or a reducing agent, processes have also become known that do not use such additions.

It is known, for example, to treat iron articles for 24 hours with a solution containing chromic acid, sodium chromatc, sodium bichromate and phosphoric acid, and subsequently to dry the articles for 1 /2 hours at 120 to 200 C. According to another known process, the metal surface is treated with the co-operation of heat with a chromic acid solution that is free from activator ions in a manner such that a chemical reaction takes place with the formation of a coating. The metal parts may be treated at least at 105 C. with a concentrated chromic acid solution having a boiling point of at least 105 C. until a visible coating has been formed on the retail parts, or the solution is applied to the metal parts in the form of a film which is burned in at elevated temperature so that a chemical reaction takes place.

All these known processes, however, which use hex- Patented avalent chromium have considerable disadvantages and, therefore, they are employed in practice not at all or to a small extent only. Some of the processes referred to, above all those using a reducing agent, are particularly unsuitable for continuous operation, while the others are either uneconomical or the protection of the treated metal parts is insuficient.

Now we have found a process for producing surface coatings on iron and steel that are corrosion-resistant and/or improve the adhesion of lacquers and varnishes, which does not possess the aforesaid disadvantages of the known processes and comprises contacting the surface of the degreased metal parts to be protected first with a dilute aqueous solution (I) of one or more aliphatic hydroxycarboxylic acids, and subsequently, if desired after having been rinsed with water, with an aqueous solution (II) of chromic acid, and rinsing the surface of the metal parts thus treated in a third step, before drying them, with water or a very dilute aqueous solution of chromic acid.

The above described process according to the invention is illustrated in the single figure of the drawing. The flow diagram there set forth shows the three step process according to one embodiment of the invention wherein the metal surface is first contacted with a hydroxycarboxylic acid, secondly contacted with a s-olu tion of chromic acid and lastly rinsed with either water or a very dilute solution of chromic acid.

As aliphatic hydroxycarboxylic acids there may be used according to the invention mono, biand tribasic hydroxycarboxylic acids, i.e., hydroxycarboxylic acids containing 1, Z or 3 COOH groups, which contain not more than 6 carbon atoms in the molecule, for example, glycolic acid, lactic acid, tartaric acid and citric acid. In particular lactic acid, tartaric acid and citric acid can be used with advantage in the process of the present invention.

According to the present invention it has proved especially advantageous to use a pretreatment bath (1) that has a total acid concentration within the range of about 0.5 to about 20% by weight, preferably 1.5 to 8% by weight. in addition to the hydroxycarboxylic acids the solutions (I) may contain phosphoric acid and/or oxalic acid and, if desired, inhibitors. Further, it is of advantage when the proportion of phosphoric acid and/ or oxalic acid to hydroxycarboxylic acid does not exceed 1. The addition of appropriate acid inhibitors is especially suitable when concentrated solutions are employed, in the case of operationally conditioned prolonged periods of treatment or when higher bath tem peratures are applied for the treatment of largely rustfree metal parts.

Suitable inhibitors are substances which even at temperatures within the range of about 60 C. to about C. satisfactory reduce the pickling action exerted by the hydroxycarboxylic acids, the oxalic acid and the phosphoric acid on the surfaces of iron and steel and which, during the regeneration of the exhausted solutions in the cation exchanger, are not absorbed. As an example of a suitable inhibitor diet.yl thiourea may be mentioned which is used in a concentration within the range of about 0.06% to about 0.25%, calculated on the total acid.

According to the invention the metal parts which have been pretreated with solution (1) containing hydroxycarboxylic acid are treated with a chroniatization solution (ii) containing chromic acid. The concentration of chromic acid in solution (II) is advantageously within the range of about 0.1 to 10% by Weight, preferably 0.25 to 3.5% by weight. However, a good protection can also be attained by treating the metal parts with solutions containing, for example, 25% by Weight of chromic acid and a small portion of alkali metal phosphate and by subsequently rinsing them thoroughly with water that is free from chromic acid.

It has been found that an unobjectonable protection can already be obtained by means of chromic acid solutions that are free from-any additives. In many cases it has, however, proved to be very suitable to add small quantities of phosphoric acid or alkali metal phosphates to the chromic acid solution (II). As phosphates there may be used in particular alkali metal ortho phosphates and alkali metal polyphosphates, for'example, Na PO K PO and Na l O The concentration of the phosphate added depends in the first line on the chromic acid concentration of the solution and, if possible, it should not exceed by weight of the content of chromic acid.

The treatment of the metal parts with solutions (I) and (II) and the subsequent rinsing step can be carried out according to the invention at temperatures within the range of about 3 C. to about 80 C., advantageously at temperatures Within the range of about 5 f C. to about C. Bath temperatures above 30 C. are in particular suitable When very rusty parts are treated for a short time.

A special advantage of the process of the invention resides inthe fact that the exhausted treatment solutions, that is to say both pretreatment solution (I) and chromatization solution (H), can be regenerated by cation exchangers, whereby the iron ions which got into solution ,(I) when the metal parts were being treated with solution (I) are removed from that solution and the iron ions and the Cr ions originating from the reduction of chromic acid are removed from solution (H), whereupon the regenerated solutions can be used again in the process of .the invention.

As comparative experiments have shown, the treated metal parts are provided with insufiicient protective coatings only when the treated metals, after hawng been degreased and then rinsed with water, are directly treated with solutions containing compounds of hexavalent chromium such, for example, as chromic acid. Even if high concentrations of chromium (VI) compounds are applied,

acids, cannot be replaced by other acids which are comrnerically available.

When, for example, hydrochloric acid or sulfuric acid is used instead of the hydroxycarboxylic acid, rust-brown coatings which do not give protection against corrosion are formed on the treated metal parts.

The possibility of using these acids is limited because they enable a protection against corrosion to be obtained only when the chromatization is carried out'immediately after the treatment with acid. In most cases this requireautomatic conveyorized mechanized operation. However, when the process is carried out according to the present invention the intervals of the individual steps occurring in a process carried out in automatic conveyorized me- Pretreatment solutions containing fatty acids, for exment cannot be met when the process is carried out'in chanical operation do not deteriorate the quality of the :coatin g protection against corrosion.

ample, acetic acid or formic acid, which likewise enable a protection against corrosion to be obtained can in general be used in special cases only, whichis due to the 7' following reason: If several minutes pass between the individual steps of "the process, as is, for example, the case when parts having large surfaces, for example, bodies of motor cars, are

treated in automatic conveyorized mechanical operation,

rust, which diminishes the protection against corrosion after the chromatization, forms on the metal surface after the latter has been pretreated with the said solutions containing fatty acids. In contradistinction thereto, the process according to the present invention enables an excellent protection against corrosion to be obtained in these cases,

too.

The method according to the present invention is suitably oarried out in the following way: The iron or steel parts are treated according to the oxydic nature of their surfaces for up to about 10 minutes with pretreatment solution (I) defined above and then, after a short intermediate rinsing with desalted water, which rinsing may be dispensed with, introduced into chromatization bath (II), which contains chromic acid and in addition thereto may contain phosphoric acid and/ or alkali metal phosphate. After having been rinsed again with a strongly diluted solution of chromic acid which should contain no more than 0.08% byweight of CrO or, if desired, with desalted water (when a chromic acid bath (ll) of higher ing the corresponding concentrates in the same measure as they are consumed. The iron ions enriched in the operating pretreatment baths can be determined in known manner and removed by ion exchangers.

The following examples serve to illustrate the invention but they are not intended to limit it thereto.

EXAMPLE 1 Degreased steel plates for bodies of motor cars were dipped for 8 minutes at a temperature of 20 C. into a solution (1) containing 2.1% by weight of lactic acid. After another 6 minutes during which time the plates hung in the air they wererinsed for 1 minute with desalted water and after having stayed for another 6 minutes in the air they were dipped at a bath temperature of 20 C. into a bath (II) containing 3.52 grams per liter of chromic acid and 0.48 gram per liter of N21 PO The sheets were then again hung for 6 minutes in the air, then aftertreated in a bath 'containing'0.21 gram per liter of chromic acid and 0.029 gram per liter of Na PO and .dried with hot air.

For comparison, degreased steel plates for bodies of motor cars were dipped at C. in the same manner as described above into a solution containing 460 grams per liter of chromic acid and having a boiling point of 105 C. After about 27 minutes for formation of a coat could be observed on these plates; the plates became dull.

The plates thus treated were finally rinsed with desalted water and dried atlSO C.

For comparison, steel plates for bodies of motor cars were sand-blasted and part of the sand-blasted plates was dipped for 60 minutes into a bath containing 240 grams per liter of chromic acid and having'a temperature of 25 C. The other part of the sand-blasted steel plates "was treated for 15 minutes with a solution containing 3.52

grams per liter of chromic acid. The sand-blasted and then chromatized plates were finally rinsed with a solution containing 0.21 gram per liter of chromic acid and then dried. 7

Both the steel plates treated by the method of the invention and those treated by the known processes for comparison were then provided with an alkyd resin baking lacquer in two stages (base lacquer and covering'lacquer) at a baking temperature of C. and with a baking time of 25 minutes and subjected to Various usual corrosion and lacquer adherence tests. The following results were obtained.

(1) Specimens treated by the method of the invention (2) Specimens treated for 27.5 minutes at 105 C. with a concentrated chromic acid solution (4-60 g./l.) (3a) Sand-blasted specimens treated for 60 minutes in a solution of 3.52 g./l. of chromic acid Note 5.0 (3b) Sand-blasted specimens treated for 15 minutes in a solution of 240 g./l. of chromic acid Note 3.5 (4) Specimens treated with dilute hydrochloric acid instead of with lactic acid, rinsed with desalted water and then treated with chromic acid according to the invention Note 1.0 to 2.5 (5) Specimens treated with Warm phosphoric acid of strength instead of with lactic acid and then further treated according to the invention Note 3.5 to 5.0

Note 6.0

Note 5.0

(4) exposed for 6 minutes to the action of air.

(5) The plates were then treated for 2.5 minutes under a spraying pressure of 0.7 atmosphere (gage) and at a bath temperature of about C. with a chromic acid solution (II) of 0.313% strength which contained 0.35 gram per liter of K HPO (6) They were then exposed for 6 minutes to the action of air,

(7) treated for 1 minute at a bath temperature of about 20 C. with a chromic acid solution of 0.03% strength by weight and (8) dried with hot air.

The specimens thus treated had an excellent lacquer adherence and a very good protection against correction.

EXAMPLE 3 Degreased steel plates for bodies of motor cars were treated according to the dipping method in the following way:

(1) They were treated with a solution containing a hydroxy-carboxylic acid.

The composition of the individual solutions and the temperatures and periods of treatment can be seen from the following Table 1:

Table I Composition of solution (I) Concen- Temperatration in Inhibitor in ture of Time of Conccnpercent by percent by treattreatment Component A tration Component B weight weight ment, C.

in percent by weight (n) Lactic acid- 18 4 seconds. (0) Lactic acid- 7.5 10 90 seconds. (c) Lactic acid 2. 1 Tartai'ic acid 15 2 minutes, ((1) Lactic acicL 1 7 30 seconds. (e) Glycolic acid. t 2. 5 18 6 minutes. (i) Lactic acid 1 18 3 minutes. (5:) Citric acid 2 l8 3 minutes. (h) Citric acid 2 0.1 of diethyl l8 6 minutes.

thiourea. (i) Lactic acid 3 H3PO4 t. 1 0.15 of diethyl 18 10 minutes.

thiourea.

(k) Lactic acid 2. 2 Osalic achl.-- 0. 8 23 7.5 minutes. (1) Glycolic acid 1. 4 Oxolic acid. 1. 2 18 6 minutes, (in) Lactic acid 0. 7 28 10 minutes.

Explanations:

Note l no protection against corrosion (the lac quer peeled off completely) Note 2=peeling oit of lacquer 1 cm. on either side Note 3=peeling off of lacquer 0.5 cm. on either side Note 4=peeling oii of lacquer 0.25 cm. on either side tote 5=peeling oil of lacquer 0.1 cm. on either side Note 6=very good protection against corrosion (no peeling off of lacquer along the scratches).

(6) The tests for the lacquer adherence according to Erichsen in accordance with DIN specification 53.156 gave the highest values of the test scale, namely 10.0 min, for the steel plates treated by the method of the invention.

EXAMPLE 2 In the same manner as described in Example 1 degreased steel plates for bodies of motor cars were treated in a spraying device as follows:

(1) They were treated for 2.5 minutes under a spraying pressure of 0.8 atmosphere (gage) and at a bath temperature of about 20 C. with a solution (I) containing 1.8% by weight of lactic acid and 0.2% by weight of tartaric acid.

(2) They were exposed for 6 minutes to the action of air.

(3) They were treated for 1 minute at 20 C. with desalted water, and

(2) The steel plates were exposed to the action of air for 4 minutes in cases (0), (d), (f) and (i), for 2 minutes and 40 seconds in cases (b), (e), (g), (h), (k) and (l), for 1 minute and 10 seconds in cases (a) and (m).

(3) The steel plates were treated with desalted water at 20 C. and then (4) exposed to the action of air as described under (2).

(5) They were treated for 4 minutes with a chromic acid solution (H) of 0.313% strength at a bath temperature of about 20 C. for the periods indicated in Table 1.

(6) They were exposed to the action of air as described under (2), then (7) treated for 1 minute at 20 C. with a chromic acid solution of 0.03% strength, and

(8) dried with hot air.

The samples thus treated were lacquered in the manner described in Example 1 and tested. They had an excellent lacquer adherence and a very good protection against corrosion.

In treatment solution (at) steel plate surface was treated until the solution became exhausted owing to a high content of iron ions. This exhausted solution could be regenerated by means of cation exchanger Lewatit S100. After this regeneration the solution could be used again in an unobjectionable manner.

EXAMPLE 4 Degreased steel plates for bodies of motor cars were dipped for 10 minutes at 20 C. into a solution (1) containing 2.4%-of lactic acid and after 3 minutes, during which period the plates hung in the air, they were rinsed :for 30 seconds at 13 C. with desalted water. After being exposed to the action of air for another 3 minutes they were treated'with a chromic acid bath (II) having the a composition indicated in Table 2, into which they were 'The test plates treated as indicated under ([2) and were exposed for another 3 minutes to the action of air 'and'then treated for a minute at about C. with desalted water. The test plates treated as indicated under ,(a), (d) and (e) were rinsed for 1 minute at about 20 C. with a chromic acid solution of 0.03% strength by weight and then dried in a current of hot air. 'After the samples had been lacquered in the manner described in Example 1 all of them had a good lacquer adherence and .a very good protection against corrosion.

We claim:

1. The method of producing'adherent uniform' coatings on iron :and steel parts, particularly for preventing corrosion and providing adherence of lacquers and varnishes, which method consists essentially of contacting the surfaceof saidmetal parts in a first step at a temperature between about 3 C. and about 80 C. with .an aqueous solution (I) containing about 0.5% by weight to about 20% by weight of at least one aliphatic hydroxycarboxylic.

acid with 1 to 3 COOH-groups and having not more than 6 C-atoms in its molecule, subsequently in a second step at a temperature between about 3 C. to about 80 C. with an aqueous solution (ID-containing about 0.1% by weight to about 10% by weight of chromic acid, and rinsing the,

' surface of the metal parts thus treated in a third step at a temperature between about 3 C. and about80 C. with at least one-liquid (III) selected from the group consisting of water and an aqueous solution containing chromic'acid in a concentration not exceeding 0.08% by weight.

.2. Themethod as described in claim 1, wherein the surfaces of said metal parts treated with said solution (I) are rinsed with water before being contacted with said solution (11).

3. Themethod as described in claim 1, wherein said 'solution'(I) contains about 1.5% by weight to about 8% by weight of said hydroxycarboxylic acids.

4. The method as described in claim -1, wherein said solution (I) additionally contains at least one acid selected from the group consisting of phosphoric acid and oxalic acid, the total amount of the acids contained in said soluo ticn not exceeding 20% by weight and the ratios of phosphoric acid:hydroxycarboxylic acid and oxalic acidzhydroxycarboxylic acid being not more than 1.

5. The method as claimed in claim 4, wherein the total amount of the acids contained in said solution (I) does not exceed8% by Weight.

6. The method as described in claim 1, wherein said solution (1) contains about 0.06% by weight to about 0.25% by weight of a pickling inhibitor, calculated on the total amount of the acids contained in said solution (I), said inhibtor reducing the pickling action of the acid contained in solution (I) on surfaces of iron and steel at temperatures between about C. to about C. and being not retained by cation exchangerswhen aqueous solutions containing said corrosion inhibitor are treated therewith.

'7. The method as described in claim 1, wherein the surfaces of said metal parts are contacted with the solutions (I) and (II) and subsequently rinsed at a temperature of about 5 C. to about 30 C.

8. The method as described in claim 1, wherein said solution (II) contains about 0.25% by weight to about 3.5% by weight of chromic acid.

9.'The method as described in claim 1, wherein said solution (II) additionally contains at least one substance selected from the group consisting of phosphoric acid and alkali metal phosphate in a quantity not exceeding 25% by weight of the amount of the chromic acid present.

10. The method as described in claim 1. wherein the iron ions of the exhausted solution (I) originating from the surfaces of said metal parts by the contacting with the solution (I) are separated by leading said exhausted solution (I) over a cation exchangen 11. The method as described in claim 1, wherein the iron ions and the CR +-ions of the exhausted solution (II) are separated by leading said exhausted solution (II) over a cation exchanger.

I References Cited by the Examiner UNITED STATES PATENTS RICHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner. 

1. THE METHOD OF PRODUCING ADHERENT UNIFORM COATINGS ON IRON AND STEEL PARTS, PARTICULARLY FOR PREVENTING CORROSION AND PROVIDING ADHERENCE OF LACQUERS AND VARNISHES, WHICH METHOD CONSISTS ESSENTIALLY OF CONTACTING THE SURFACE OF SAID METAL PARTS IN A FIRST STEP AT A TEMPERATURE BETWEEN ABOUT 3*C. AND ABOUT 80*C. WITH AN AQUEOUS SOLUTION (I) CONTAINING ABOUT 0.5% BY WEIGHT TO ABOUT 20% BY WEIGHT OF AT LEAST ONE ALIPHATIC HYDROXYCARBOXYLIC ACID WITH 1 TO 3 COOH-GROUPS AND HAVING NOT MORE THAN 6 C-ATOMS IN ITS MOLECULE, SUBSEQUENTLY IN A SECOND STEP AT A TEMPERATURE BETWEEN ABOUT 3*C. TO ABOUT 80*C. WITH AN AQUEOUS SOLTUION (II) CONTAINING ABOUT 0.1% BY WEIGHT TO ABOUT 10% BY WEIGHT OF CHROMIC ACID, AND RINSING THE SURFACE OF THE METAL PARTS THUS TREATED IN A THIRD STEP AT A TEMPEATURE BETWEEN ABOUT 3*C. AND ABOUT 80*C. WITH AT LEAST ONE LIQUID (III) SELECTED FROM THE GROUP CONSISTING OF WATER AND AN AQUEOUS SOLUTION CONTAINING CHROMIC ACID IN A CONCENTRATION NOT EXCEEDING 0.08% BY WEIGHT. 